I didn't ask. As I understand it, it's less about legality and more about plausible deniability; on a "party boat" with plenty of other public people to make it cheaper than renting a whole boat, plus the week for the cruise and time to relax -- seems plausible that these people "just happened" to book the same boat at the same time at peak tourist season and decide to throw a "private party". I should have asked more questions, but there were plenty of other people to chat up.